added support for multiple parents

[JalexChen]

• added 3 new variables to manifest: -- parentTags supports Elixir and Clojure, allowing them to choose versions of Erlang and OpenJDK, respectively -- defaultParentTag tracks the default tag for new version updates -- parentSlug helps to differentiate from diverging repository/parent names and to help track for Dockerfile builds

• Refactored parser to take in variables and account for all cases

• Added build and push function to help generate Docker strings

• waiting on: https://github.com/CircleCI-Public/cimg-clojure/pull/18 https://github.com/CircleCI-Public/cimg-elixir/pull/71

• Closes #51

• Closes #60

[FelicianoTech]

Also, when parentTags are activated, we need to disable:

1. the creation of the Dockerfiles that are outside of parentTags
2. the building of those Dockerfiles in the build file
3. the pushing of those in the push file (although maybe this part is fine, since those tags should still exists)